1. Field of the Invention
The present invention relates to injection moldable poly(ethylene terephthalate) resin (PET) compositions containing reinforcing glass fibers. More specifically, the invention is directed to reinforced PET resins which have unexpectedly smooth surfaces when molded.
2. Discussion of the Background
Poly(ethylene terephthalate) resins have been known and commercially available since the mid 1960's. PET resins have useful barrier properties and can be easily injection molded and reinforced with glass fiber materials.
A continuing problem associated with the use of PET resins is the ability to achieve satisfactory parts having good surface smoothness from glass reinforced PET resins at molding temperatures below about 110.degree. C. One approach to this problem has been the use of mold releasing agents to facilitate release of the molded resin from the mold.
Other attempts to solve the surface smoothness problem have centered on the difficulties arising from the slow crystallization rate of the polymers at temperatures below about 110.degree. C. Crystallization promoting agents such as polyoxyalkylene chains having pendant epoxy groups and particulate nucleating agents have been utilized to enhance the crystallization rate of the glass fiber reinforced PET resins to allow the material to be molded in molds heated to temperatures less than 110.degree. C. and yet maintain a smooth glossy finish.
U.S. Pat. No. 3,435,093 discloses the use of poly(ethylene terephthalate) resins containing ionic hydrocarbon copolymers of an .alpha.-olefin, and .alpha.,.beta.-ethylenically unsaturated carboxylic acids which have been at least partially neutralized. The acid groups of the ionic copolymer are randomly distributed along the polymer chain and can be neutralized from 0% to 100% with metal cations such as Na, K, Ca, Mg, Zn, and Pb.
U.S. Pat. Nos. 4,486,564 and 4,352,904 disclose PET blends containing, in general, from about 0.5 to 12 weight % of the salt of an ethylene/methacrylic acid copolymer as well as about 1 to 12 weight % of a low molecular weight organic ester, ketone, sulfone, sulfoxide, nitrile, or amide. The low molecular weight compounds are required to obtain a high surface gloss and function by improving the mobility of the PET resin in its supercooled state by reducing the viscosity of the supercooled mixture. Both the ethylene/methacrylic acid copolymer salt and the low molecular weight mobility additive are required to obtain a high surface gloss and smoothness.
A need still exists, however, for an inexpensive simple method of producing glass reinforced poly(ethylene terephthalate) resins with smooth and glossy surfaces at mold temperatures less than about 110.degree. C.